Modify ASP.NET Owin Identity User

I am attempting to add a few fields to the Identity User for the Owin Identity, like this:

public class ApplicationUser : IdentityUser
{
    public async Task<ClaimsIdentity> GenerateUserIdentityAsync(UserManager<ApplicationUser> manager)
    {
        var userIdentity = await manager.CreateIdentityAsync(this, DefaultAuthenticationTypes.ApplicationCookie);
        return userIdentity;
    }

    public DateTime BirthDate { get; set; }
}

However, I get this error:

Mapping and metadata information could not be found for EntityType 'Code.ApplicationUser'.  I need help understanding this and how to fix it.

Thank you,